Inductive circuits are an essential component of radio frequency circuits which are used particularly in the field of mobile telephones. An inductive circuit is an element of an inductive/capacitive tuned resonant circuit. Inductive/capacitive tuned circuits are, in particular, used in tuned radio frequency amplifiers (generally bandpass radio frequency amplifiers).
One particularly sensitive feature of these amplifiers resides in the selectivity of the inductive/capacitive resonant circuit. This is so since outside the working frequency band of the amplifier, all the other spectral components are considered as noise. Conventionally, the frequency response of an inductive/capacitive tuned amplifier contains a peak which is centered on the resonant frequency FO and has a width at the half-height commonly denoted by .DELTA.f. The resonant frequency FO is equal to the inverse of the square root of the product of the inductance times the capacitance.
The ratio FO/.DELTA.f is referred to as the "quality factor" of the inductive/capacitive resonant circuit. In the rest of the text, and by a convenient oversimplification, the term "quality factor" will be associated with the inductive circuit on its own. This quality factor should be as high as possible. However, the width .DELTA.f of the resonance peak is directly proportional to the energy losses of the resonant circuit. Consequently, the higher the losses are, the more the quality factor is reduced.
On a silicon substrate, the inductive circuits are produced by forming a metal spiral which rests on the silicon on an insulating layer, typically silicon dioxide having, for example, a thickness of 1 micron. However, unlike gallium arsenide (GaAs) semiconductor substrates, silicon substrates have low resistivity. The result of this is consequently that the magnetic field generated by the flow of current in the metal turns induces very high eddy currents in the underlying silicon substrate. Some of the energy of this magnetic field will therefore be dissipated in the form of heat, consequently reducing the value of the quality factor.
To address this problem, it has in particular been proposed to use inductive circuits with a high quality factor which are external to the integrated circuit containing the other elements of the tuned amplifier. Notwithstanding, such an approach requires extra components and support which are incompatible with low production costs. Furthermore, stray interference can impair the operation of the amplifier, in particular because of the interconnections between the integrated circuit and the external inductive circuit.
It has therefore been found particularly advantageous to arrange all the components of the radio frequency amplifier, and, in particular, all the passive components, such as inductors and capacitors, in the same integrated circuit. In this regard, it has been proposed to make selective localized substrate recesses under the inductor zones. This is done by localized chemical attack or etching, particularly using potassium hydroxide (KOH).
Unfortunately, such an approach requires specific infrared masks arranged on the rear face of the substrate whose alignment with the components arranged on the front face is particularly difficult. It also entails problems in coating the chip with resin, because of the presence of these relatively large cavities in the substrate.
Another advocated approach includes fully removing the silicon substrate and replacing it with a glass substrate. This approach also has a large number of drawbacks, in particular because of the difference between the expansion coefficients of silicon and the glass, the fragility of the substrate, and the difficulty of welding and coating the chip with resin.
It has further been proposed, in an article by Y. H. Xie et al., entitled "An Approach For Fabricating High Performance Inductors On Low Resistivity Substrates", IEEE BCTM 5.3, September 1997, pp. 88-91, to produce an inductive circuit on a silicon substrate which is partly porous, so as to increase its resistivity. More precisely, a silicon substrate, typically having a thickness of 300 microns, is subject to anodic oxidation in an aqueous solution of hydrofluoric acid having a concentration of 20% by volume with an anodic current density equal to 50 mA/cm.sup.2. This makes the silicon porous to a thickness between 50 and 250 microns. An insulating layer of silicon dioxide is then deposited on the outer surface of the substrate, and covered with a metal spiral so as to form the inductive circuit.
However, the method described in this prior art document is applicable only to the production of an inductive circuit. It is unsuitable for the simultaneous production, on the same porous silicon substrate, of other active components, such as, for example, bipolar transistors and/or complementary field-effect transistors with insulated gates (CMOS transistors). These are typically needed for producing the other elements of an integrated tuned radio frequency amplifier. Indeed, this document indicates that the internal surface of the pores of the porous silicon is a strong source of contamination. This is so in particular for the gas atmospheres of the ovens which would be used to produce the bipolar or CMOS transistors on this same porous silicon substrate. What is more, further to these problems of contamination, the porous silicon undergoes surface deformations when hot, and this is particularly unsuitable for the production of bipolar and/or CMOS transistors prior to this phase of converting silicon into porous silicon.